We bought our house about 6 yrs ago and had hardwood, white oak, put in prior to moving in. This summer I started to replace the white composit trim with 3" wood trim. I ran into 2 issues and I hope someone on here can help. The first issue is not all of the walls are straight or flat but rather have some concave or curve to them. I'm guessing this is normal for a house built in '94? Anyway, I can't get the trim to follow the curve of the walls. I'm sure I could if I used screws but I don't think that would be the right way, if I'm wrong I'm sure someone will tell me so. When I press the trim to the wall and nail it in it just straightens out when I remove the pressure. Is there a way to solve this issue? I did not use liquid nails and doubt it would work based on how much pressure I have to apply to get the trim to bend in some places.

The other issue I have is that apparently my floor is also not flat or level. Again I tried to apply downward pressure on the trim boards but then my cuts were out of whack.

Any thoughts on one or both issues would be greatly appreciated.

EDIT: So it looks like the pics are not going to display so here are links.
Concave wall issue:http://flic.kr/p/gUGY3q

Going to have to find the studs and nail there. Use 3" galvanized finish nails for more holding power. For sure there's going to be something to nail to at the very bottom where the bottom plate is so nail there first.
As bad as those walls are you may even need a few finish head screws.
There's still going to be smalls gaps with that type texture.
Some caulking will hide them. I only use Alex 230.
The trick is to make a small hole in the nozzle and only enough pressure to apply a small amount.
Have a pan of water handy to clean your fingers and wipe the area to clean up the access.
There only needs to be caulking in the gap not all over the moulding and wall.
Floors are never flat, your going to have to used 1/4 round or shoe moulding.
And no it not common for a wall to be that far out, looks like someone used a crooked bottom plate.

Joe has covered this pretty well I will just add, toe nails into framing can't pull straight back out. If you don't like the shoe molding, you tack the molding to the wall level and scrib it to the floor and cut to fit, after five ft of that you will like the shoe molding a lot more.

Last point ... instead of forcing the larger molding to fit the wavy floor, the historic solution is to add another course of shoe molding. Just nail your larger, thicker molding as level as you can. Then, add the smaller, more flexible, molding to the bottom.

You can also add a similar molding to the top in lieu of a thick caulk line. Then you will be left with a much thinner caulk line on top and no gap at the bottom.